1. Field of the Invention
This invention relates to hollow shafts of turbines, and more particularly, to means for heating the hollow shaft from its inside to its outside.
2. Description of the Prior Art
When large rotating apparatus such as steam turbines which normally operate at elevated temperatures are removed from service, access to the turbine by the steam or other heating medium is usually prevented and the turbine is either brought to rest or rotated at low RPM's by turning gears. To safely and reliably accelerate the steam turbine to synchronous speed and bring it under load, the turbine parts which are normally exposed to the hot, motive steam must be increased in temperature at a relatively slow rate to avoid high internal material stresses from being experienced. Such slow temperature increases are especially critical for the turbine's shaft since, in addition to the high thermal stresses experienced by it, high, centrifugally induced stresses are imposed upon it. To assure quick response to increasing load demand it has often been necessary for utilities to maintain an abnormally high spinning reserve of generating capacity. Such spinning reserve requires capital expenditures for additional equipment and often results in high operating costs for that equipment due to its relatively low efficiency at partload, normal demand operation. Reduction in the amount of spinning reserve necessitates having the capability of quickly accelerating steam turbines from turning gear operation to operating speed while avoiding high internal material stresses during such fast rotative acceleration and material temperature elevation.
U.S. Pat. No. 2,004,777 which issued June 11, 1935, discloses electrical heating of turbine casings and other associated stationary parts such as flanges. U.S. Pat. No. 1,811,383 which issued June 23, 1931, illustrates a system for preheating turbine components by continuously passing heating steam therethrough and removal of moisture droplets condensed within the turbine. Both of the aforementioned patents illustrate prior attempts to reduce turbine startup time by preheating selected parts of the turbine. Neither patent, however, illustrates heating the turbine shaft from the inside toward the outside. It can be shown that the shaft material near the bore experiences the highest centrifugal force stress loading and neither of the aforementioned patents provide means for directly heating the material in such region. Further attempts to heat turbine shafts from their inside include routing heating steam to the inside bore, but it has been found that such practice can promote stress corrosion cracking of the turbine shaft and is thus considered undesirable. It is to be noted that U.S. Pat. No. 2,004,777 illustrates electrical heating of the casing elements only and provides no means for preheating the rotatable shaft element. A further disadvantage of U.S. Pat. No. 1,811,383 is that it requires a continuous steam flow through the turbine to maintain the desirable temperature in all turbine parts. Such practice is expensive since it requires constant expenditure of heat energy to maintain turbine component temperatures at acceptable levels.
Supplying electrical energy to electrical heaters disposed within turbine shafts has heretofore presented a variety of problems. Ser. No. 857,480, filed Dec. 5, 1977, provides a solution to many of the problems that previously existed for conducting electricity from a stationary source to the turbine shaft's interior. Since such conduction is now practical, it is desirable to obtain an electrical turbine bore heating system which uses relatively simple electrical heaters which can be easily assembled in turbine shafts, will provide uniform heating, and will have heating capability sufficient to rapidly heat the turbine shaft from its interior.